This invention relates generally to field effect transistors and, more particularly to field effect transistors which operate at microwave frequencies.
As is known in the art, there are several types of field effect transistors (FETs) generally used at microwave and millimeter wave frequencies. These transistors include metal semiconductor field effect transistors (MESFETs) and high electron mobility transistors (HEMTs) each fabricated from Group III-V materials. What distinguishes a HEMT from a MESFET is that in a HEMT charge is transferred from a doped charge donor layer to an undoped channel layer, whereas in the MESFET the channel layer is doped and thus provides its own carriers. Generally, in HEMTs the charge donor layer is a wide bandgap material such as aluminum gallium arsenide, whereas the channel layer is of a lower bandgap material.
There are in addition two general types of high electron mobility transistors. One type is referred to simply as a HEMT, whereas the other type is a pseudomorphic HEMT. The difference between the HEMT and the pseudomorphic HEMT (PHEMT) is that in the pseudomorphic HEMT one or more of the layers, generally the channel layer, has a lattice constant which differs significantly from the lattice constant of the other materials providing the other layers in the device. Thus due to a resulting lattice constant mismatch, the crystal structure of the material of the channel layer is elastically strained.
As mentioned above, in a HEMT the doped charge donor layer is comprised of a wide bandgap material such as gallium aluminum arsenide, whereas the channel layer is typically comprised of a lower bandgap material such as gallium arsenide.
In the pseudomorphic HEMT, the undoped gallium arsenide channel layer is replaced by a channel layer comprised of a lower bandgap material such as gallium indium arsenide. Indium arsenide had a lattice constant which is substantially different than the lattice constant of either gallium arsenide or aluminum arsenide. An indium gallium arsenide channel layer provides a layer of a material having a lattice constant which is substantially larger than the lattice constant of gallium arsenide or gallium aluminum arsenide. This lattice constant mismatch makes practical growth of PHEMTs difficult and otherwise limits several advantages which would have accrued to a device using gallium indium arsenide as a channel layer. For example, the use of gallium indium arsenide as a channel layer would provide a device having several performance advantages over a simple gallium arsenide channel layer. One advantage relates to the lower bandgap characteristic of gallium indium arsenide which provides a larger conduction band discontinuity at the gallium aluminum arsenide/gallium indium arsenide heterojunction, compared to that of a gallium aluminum arsenide/gallium arsenide heterojunction. Accordingly, the charge density transferred into the GaInAs channel layer is higher because of the larger discontinuity. Moreover, gallium indium arsenide also has a higher electron mobility and higher saturated electron velocity than gallium arsenide. Each of these benefits thus can provide a pseudomorphic HEMT which theoretically can handle higher power levels, as well as, operate at higher frequencies with improved noise properties when compared to a high electron mobility transistor using a conventional gallium arsenide channel layer. These benefits also should increase with increasing indium concentration in the gallium indium arsenide layer.
Accordingly, a major objective in fabricating a high performance pseudomorphic HEMT structure is to maximize the amount of indium contained in the gallium indium arsenide layer. A problem arises, however, in increasing the indium concentration. As mentioned above, gallium indium arsenide has a lattice constant which is larger than the lattice constant of gallium arsenide or gallium aluminum arsenide with the latter two having substantially equal lattice constants. This disparity in lattice constants increases with increasing indium concentration. Thus, when gallium indium arsenide is disposed over gallium arsenide, the film develops intrinsic strains. For in-plane atoms, the strains are compressive whereas for atoms perpendicular to the growth plane the strains are tensile. This arrangement is commonly referred to as tetrahedral distortion.
For a gallium indium arsenide layer which is thicker than the so-called "critical thickness" for the particular composition of the gallium indium arsenide layer this intrinsic strain causes the gallium indium arsenide film to be disrupted with formation of various types of crystal dislocations or defects. The presence of such crystal dislocations or defects seriously degrades the electron transport properties of gallium indium arsenide layer. For a gallium indium arsenide layer having a thickness less than the so-called critical thickness of the layer, the material is elastically strained without such dislocations forming. In the growth plane, the gallium indium arsenide takes on the lattice constant of the underlying gallium arsenide or gallium aluminum arsenide layer, whereas the crystal of the gallium indium arsenide is deformed such that in a plane perpendicular to the growth plane the crystal has expanded. This type of layer is termed pseudomorphic from which is developed the term pseudomorphic HEMT. With increasing indium concentration, the critical thickness at which the gallium indium arsenide layer forms crystal defects decreases. For example, for a channel layer comprised of gallium indium arsenide having the concentration Ga.sub.0.8 In.sub.0.2 As a layer thickness of approximately 100 .ANG. is the maximum thickness. Layers thinner than 100 .ANG. are not attractive due to the increased importance of the quantum size effect which reduces the effective bandgap discontinuity and thicknesses much above 100 .ANG. can result in the above-mentioned lattice dislocation problem.
One approach to addressing the problems resulting from the lattice mismatch between gallium indium arsenide and aluminum gallium arsenide is to grow gallium indium arsenide alloys or layers on indium phosphide substrates. The lattice matched composition of gallium indium arsenide on indium phosphide substrates is Ga.sub.0.47 In.sub.0.53 As. For some applications, this is an acceptable arrangement. However, other applications require additional material improvement. There are problems experienced with the gallium indium arsenide indium phosphide system. One problem is that in a high electron mobility structure incorporating gallium indium arsenide as a channel layer, an aluminum indium arsenide charge donor layer having a composition of Al.sub.0.48 In.sub.0.52 As, which lattice matches to the indium phosphide, is also used as the layer over which the Schottky barrier electrode is placed for field effect and high electron mobility transistor types. The bandgap of such a material is 1.5 eV compared to 1.8 eV for gallium aluminum arsenide (Ga.sub.0.75 Al.sub.0.25 As) which is used as a Schottky barrier contact material for conventional HEMTs grown on gallium arsenide substrates. The lower bandgap of the aluminum indium arsenide layer in addition to the small bandgap of gallium indium arsenide reduces the breakdown voltage and consequently limits the power performance for devices based on indium phosphide substrates. Moreover, in the indium phosphide system, aluminum indium arsenide is used as the buffer layer as well as a charge donor layer in the Schottky barrier layer. However, high material quality of aluminum indium arsenide is difficult to achieve. This is because aluminum arsenide is best grown at high temperatures, whereas indium arsenide is best grown at low temperatures. The resulting compromise in growth temperatures leads to a compromise in material quality. Another problem is that indium phosphide substrates are considerably less thermally stable than gallium arsenide which is a further limitation on useable growth temperatures. Indium phosphide substrates are also more fragile than gallium arsenide leading to difficulties in handling and fabricating devices over the indium phosphide substrates. Further, indium phosphide substrates are doped with iron to provide substrates having semi-insulating properties. Iron is known as a fast diffuser in gallium arsenide. Thus, there exist a danger that iron may migrate from the indium phosphide substrates into the active layers of the transistor. Iron present in the active layers of the field effect transistors would seriously degrade their performance characteristics, since iron forms deep carrier traps.